Wright.] 
144 
[January 19, 
terrace, fifteen or twenty feet. The river may have flowed upon 
the surface of the gravel in a shallower current than the terrace 
would seem to imply. 
But when the current, passing down this declivity of four feet 
to the mile, reached the level of the sea at Trenton, its transport- 
ing power would he greatly diminished and thus we should have 
an accumulation of gravel at the head of tide water, without 
bringing into the problem the supposition of any very extraor- 
dinary increase in the volume of the river. The loss of trans- 
porting power upon diminishing the rapidity of a current of 
water is enormous. The transporting capacity of a stream of 
water is estimated to vary as the sixth power of the velocity, i.e., 
if a current is checked so that it moves at only half its former 
rate, its transporting capacity is diminished to one sixty-fourth. 
It is easy to see that the sudden enlargement of the valley just 
above Trenton, as well as the occurrence there of tide water, 
would diminish the rapidity of the river and hence cause an 
extraordinary deposition of gravel when it was abundant above. 
The most likely time for this deposition to have occurred was 
near the very close of the glacial period, when the lower moraines 
were fresh and when ice fields still lingered in the southern 
valleys of the Catskills. The process of deposition must have 
been so rapid that it could not have been much subsequent to the 
withdrawal of the continental glacier north of the Catskills. The 
time required for the river under present conditions to erode the 
channel it now occupies was of much greater duration. 
I hope another season to devote a month or two to further 
investigations and will now but briefly indicate what seems very 
probable and what is still in doubt. 
1. It seems altogether probable that the Philadelphia brick 
clay was deposited during the height of the Champlain epoch 
when the Delaware valley was considerably depressed below 
its present level. 
2. Towards the close of that period when the land had 
resumed nearly its present level and the ice had nearly all disap- 
peared south of the Catskills, the still swollen stream brought 
down the superabundant loose material from the kames and 
moraines and deposited it in the valley below. The material was 
